1. Field of the Invention
The present invention relates generally to electrical transformers and, more particularly, to current transformers. The invention also relates to trip devices including one or more current transformers.
2. Background Information
Trip devices are often used to protect the primary conductors of a power circuit. Trip devices typically include trip circuitry and a number of current transformers. The current transformers may supply electrical power to the trip circuitry and/or provide a signal to the trip circuitry corresponding to the amount of current flowing within the primary conductors. The current transformers interface with the primary conductors of the power circuit and transform a relatively high magnitude current flowing within the primary conductors into a relatively low magnitude current for use by the associated trip circuitry.
FIG. 1 is a simplified illustration of a known current transformer 70 which includes a high permeability core 72 that surrounds a conductor 78 of a power circuit. A portion 72a of the core 72 passes through a coil 74 (which is shown cut away in FIG. 1). A relatively small current, which may be supplied to other circuitry (not shown) via lead lines 75, is produced in the coil 74 when a relatively large current flows in conductor 78. The small current produced in the coil 74 is substantially linear over a wide operating range. Thus, as the current in conductor 78 increases, current in excess of that needed for sensing and power is generated in the coil 74. Undesirably, this excess current must be dissipated by the current transformer 70 or other devices (not shown).
FIG. 2 is a simplified illustration of another known current transformer 80 incorporating shunt 86 with an air gap 88. The current transformer 80 includes a high permeability core 82 that surrounds the conductor 78 of the power circuit. A portion 82a of the core 82 passes through a coil 84 (which is shown cut away in FIG. 2). When a relatively small current flows in conductor 78, the flux in the core 82 is low enough to avoid saturation, and air gap 88 keeps the flux in the core 82 and out of the shunt 86. As the primary current increases, the flux in the core 82 (due to the presence of the parallel magnetic paths) reaches a point where it saturates. At this point, the increase in flux with the primary current is dramatically reduced. As a result, the increase of the current in coil 84 is reduced (i.e., no longer increases). Accordingly, the current response of the current transformer 80 is substantially non-linear and is generally determined by the shape and size of the air gap 88. The transformer 80 includes electrical leads 85 for supplying the current developed in the coil 84 to other circuitry (not shown).
Thus, a need exists for an improved current transformer. There also exists a need for an improved trip device including a current transformer.